1. Field of the Invention
A soft and supple skin has a marked cosmetic appeal and is an attribute of normal-functioning epidermis. The outer layer of the epidermis, or stratum corneum, can become brittle, dry, and flaky, due to loss of water. Emollients, such as fats, phospholipids, sterols, and the like were at one time thought to be substances essential to the maintenance or restoration of softness and flexibility of the skin. More recently however, investigators have become aware that dry-appearing and flaky skin is the result of loss of water-soluble natural substances, mostly humectants, from the skin. Loss of water-soluble substances from the skin can occur when the skin is exposed to adverse conditions. When this happens the stratum corneum, although it is somewhat hygroscopic and absorbs water vapor, cannot retain moisture, and soon loses it if the atmosphere becomes drier. In any event, the stratum corneum which has lost humectants does not ordinarily absorb and retain sufficient moisture for restoration to a normal condition, and the need is evident for an artificial moisturizing system for persons who suffer from dry, chapped and flaky skin, or from dry and brittle hair and nails.
2. Discussion of the Prior Art
I. H. Blank, in J. Invest. Dermatol., vol. 18, 433 (1952), presented evidence that water is the only plasticizer of skin, and that a diminished water content results in a cornified epithelium having the undesirable properties of dryness, hardness, and brittleness.
Largely as a result of the work of Blank, mentioned above, there has been investigated many factors relating to the relationship of the water content of the stratum corneum to skin flexibility and plasticity. Information obtained during the two decades following Blank's work has been summarized by B. Idson, in Drug, Cosm. Ind. vol. 104(6), page 44, vol. 105(1) page 48, and vol. 105(2), page 48, all published in 1969.
The ability of normal healthy stratum corneum to retain water has been ascribed to the presence therein of water-soluble substances having strong hygroscopic properties. Many such substances occur together in the stratum corneum and collectively have been termed the "natural moisturizing factor" by O. K. Jacobi, in Proc. Sci. Sect. T.G.A. vol. 31, 22(1959) and in J. Soc. Cosm. Chem. vol. 18, 149 (1967).
Most of the water-soluble substances comprising the natural moisturizing factor have been identified by H. W. Spier and G. Pascher in the text "Aktuelle Probleme der Dermatologie", vol. I, pp. 1-46, published by S. Karger AG, Basel/New York, 1959.
More recently, G. Smeenk and A. M. Rijnbeek in Acta derm-venereol., vol. 49, 476 (1969) have published information indicating that the hygroscopic properties of the water-soluble corneum fraction are due to the simultaneous presence, in physiological proportions, of the components reported by Spier and Pascher, mentioned above.
It is known that a lipid membrane or covering in the stratum corneum hinders the extraction of the water-soluble substances therefrom by water, as reported by E. J. Singer and L. J. Vinson in Proc. Sci. Sect. T.G.A. vol. 46, 29 (1966), and by I. H. Blank, J. Invest. Dermatol., vol. 21, 259 (1953).
G. Sessa and H. Weissmann, in J. Lipid Res., vol. 9, 310 (1968), describe liposomes and discuss the rate of diffusion of ions across the lipid layers thereof.
U.S. Pat. No. 3,231,472 discloses the use of a synthetic skin moisturizer prepared by condensing an amino acid with a reducing sugar to provide an N-glycoside and thereafter contacting the N-glycoside with a proton donor.